Sources of radiation and other nuclear material that might be clandestinely transported across national boundaries must be found. The sources of radiation and clandestine nuclear material may be in the form of “dirty bombs” (e.g., a conventional explosive combined with radioactive nuclides designed to spread radioactive contamination upon detonation), fissile material, and other neutron and radiation emitting sources that may present a hazard to the public. During recent years, the United States government has placed mobile vehicles at strategic areas with gamma ray detectors dedicated to the task of finding fissile material. “Fissile material” includes those radioactive isotopes essential for nuclear explosives, and other isotopes found in conjunction with such radioactive isotopes.
Atomic explosives may be made from 235U, a rare, naturally occurring, isotope of uranium that lives almost 109 years, or 239Pu, a reactor-made isotope that lives more than 104 years. 235U decays with the emission of gamma ray photons (also referred to as ‘gammas’), principally at 185.6 keV and 205.3 keV. 239Pu emits a number of gamma rays when it decays, the principal ones being at 375 keV and 413.7 keV. These gamma rays are unique signatures for the respective isotopes. But fissile material invariably contains other radioactive isotopes besides those essential for nuclear explosives. For example, weapons grade uranium may contain as little as 20% 235U; the rest of the uranium consists of other isotopes. The other uranium and plutonium isotopes reveal their presence by gamma rays emitted by their daughters. For example, a daughter of 238U emits a high energy gamma ray at 1,001 keV; a daughter of 232U, an isotope present in fissile material made in the former USSR, emits a very penetrating gamma ray at 2,614 keV; and a daughter of 241Pu emits gamma rays of 662.4 keV and 722.5 keV.
It may also be desirable to detect various other radioisotopes, or signatures of such radioisotopes, that may be present in a “dirty bomb”. Detecting various isotopes of particular elements, such as cobalt or cesium, may be particularly valuable in attempting to deter terrorist threats.
U.S. Pat. No. 6,347,132, to Annis, describes seeking to detect nuclear weapons materials using an x-ray inspection system. However, Annis teaches that, in order to do so, one processes an x-ray transmission signal based on illuminating x-rays that traverse an inspected object, and, on the basis of characteristics (such as the spatial frequency of features, namely, how diffuse or compact they are) of the transmission image, in conjunction with the absence of scattering of the same illuminating radiation from certain regions, one infers that nuclear materials might be present.